γ-Aminobutyric acid-, glycine-, and glutamate-immunopositive boutons on mesencephalic trigeminal neurons that innervate jaw-closing muscle spindles in the rat: Ultrastructure and development

Authors

  • Sang Kyoo Paik,

    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
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  • Myung Kyw Kwak,

    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
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  • Jin Young Bae,

    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
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  • Hyun Won Yi,

    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
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  • Atsushi Yoshida,

    1. Department of Oral Anatomy and Neurobiology, Graduate School of Dentistry, Osaka University, Osaka 565-0871, Japan
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  • Dong Kuk Ahn,

    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
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  • Yong Chul Bae

    Corresponding author
    1. Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, Daegu 700-412, Korea
    • Department of Oral Anatomy and Neurobiology, School of Dentistry, Kyungpook National University, 188-1,2-Ga, Samdeok-Dong, Jung-Gu, Daegu 700-412, South Korea
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Abstract

Unlike other primary sensory neurons, the neurons in the mesencephalic trigeminal nucleus (Vmes) receive most of their synaptic input onto their somata. Detailed description of the synaptic boutons onto Vmes neurons is crucial for understanding the synaptic input onto these neurons and their role in the motor control of masticatory muscles. For this, we investigated the distribution of γ-aminobutyric acid (GABA)-, glycine-, and glutamate-immunopositive (+) boutons on Vmes neurons and their ultrastructural parameters that relate to transmitter release: Vmes neurons that innervate masseteric muscle spindles were identified by labeling with horseradish peroxidase injected into the muscle, and immunogold staining and quantitative ultrastructural analysis of synapses onto these neurons were performed in adult rats and during postnatal development. The bouton volume, mitochondrial volume, and active zone area of the boutons contacting labeled somata (axosomatic synapses) were similar to those of boutons forming axoaxonic synapses with Vmes neurons but smaller than those of boutons forming axodendritic or axosomatic synapses with most other neurons. GABA+, glycine+, and glutamate+ boutons constituted a large majority (83%) of all boutons on labeled somata. A considerable fraction of boutons (28%) was glycine+, and all glycine+ boutons were also GABA+. Bouton size remained unchanged during postnatal development. These findings suggest that the excitability of Vmes neurons is determined to a great extent by GABA, glycine, and glutamate and that the relatively lower synaptic strength of axosomatic synapses may reflect the role of the Vmes neurons in modulating orofacial motor function. J. Comp. Neurol. 520:3414–3427, 2012. © 2012 Wiley Periodicals, Inc.

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